Flashing Glow Plug Light: Here Is What You Need to Do

Tim Miller
Glow Plug Light


If the light stays on or flashes while you are driving, it indicates there’s a problem with the glow plugs or the electrical system.

Glow Plug Light


  • Faulty Glow Plug
  • Malfunctioning Sensor
  • Faulty Glow Plug Relay
  • Electrical Issues



Easy to fix


What Is Glow Plug?

A glow plug is an electrical heating device that helps your diesel engine start in cold weather conditions.

Diesel engines work by compressing air in the cylinders, which heats up the air. Fuel is then injected into the heated air to be ignited and power the engine.

However, in cold temperatures, the heat may dissipate quickly, making it difficult for the air to get hot enough to ignite the fuel. That’s why glow plugs come in. They function like heaters (heat up to approximately 850 – 1000°C), making the air warm enough to ignite the fuel and start the engine. After that, the glow plugs will turn off.

Nowadays, modern diesel engines use an ECU (engine control unit) to determine when to activate the glow plugs based on the engine’s temperature. This ensures that the engine only uses the glow plugs when necessary, improving efficiency and reducing wear and tear on the plugs.

What Does It Mean When the Glow Plug Light Comes on?

glow plug light

The glow plug light, also called pre-glow or wait-to-start light, shows the operation of glow plugs on your vehicle. When the light comes on, it means the glow plugs are operating to help start your engine. After the engine starts, the glow plugs stop working and the light will go off.

However, if the light stays on or flashes while you are driving, it indicates there’s a problem with the glow plugs or the electrical system. In this case, you should have your car checked to determine the issue and make any necessary repairs.

Can You Still Drive With the Glow Plug Light?

Yes, but you shouldn’t drive with the glow plug light. In this case, your car might go into “safety” mode to prevent any damage to the engine. This can cause a drop in performance. It is not a big deal if you’re driving short distances, but it could be dangerous if you’re on the highway.

Continuing to drive in this condition can cause further damage to the engine and potentially lead to a breakdown. It’s best to have your vehicle checked and refrain from driving until the issue is resolved.

Why Does the Glow Plug Light Come On After Starting?

1. Malfunctioning Glow Plug
malfunctioning glow plug

A faulty glow plug is the most common cause of the glow plug light illuminating in your diesel vehicle. When the ECU senses that one or more glow plugs are not working properly, it will turn on the glow plug light to alert the driver of the issue.

There are various reasons why glow plugs can malfunction, including:

  • Age and wear: As glow plugs age, they can become less effective at heating up, which may eventually cause them to fail.
  • Overheating: If the engine overheats, the glow plugs can be damaged and fail.
  • Contamination: When dirt or debris accumulates in the combustion chamber, it can build up on the glow plugs and cause them to stop working correctly.
2. Malfunctioning Sensor
The sensor in the control module is responsible for monitoring engine temperature and sending signals to the control module to activate the glow plugs if necessary.

If the sensor malfunctions, it may send incorrect signals to the control module, turning the glow plugs on unnecessarily. This can cause the glow plug light to come on.

3. Faulty Glow Plug Relay
The glow plug relay is responsible for supplying power to the glow plugs.

When the glow plug relay is not functioning properly, it may not supply enough power to the glow plugs, which can cause them to fail or operate at reduced efficiency. This can cause the glow plug light to come on.

4. Electrical Issues
Over time, wires or electrical connectors become loose or damaged. This affects the operation of glow plugs and triggers the light to come on.

What To Do When the Glow Plug Light Comes On After Starting

what to do when the glow plug light comes on

Step 1: Stop driving and let the engine cool down

Step 2: Check the glow plugs

  • Locate the glow plugs on your engine and remove them using a wrench or socket.
  • Check each plug for physical damage, such as cracks or breaks. Also, check the tip of the glow plugs for signs of wear or damage.
  • Use a multimeter to test the electrical resistance of each glow plug. Typically, the resistance reading for a glow plug can range between 0.5 and 2.5 ohms, but it’s best to check the specifications to confirm the correct range for your engine.
  • If you find any damaged or worn glow plugs or ones that do not meet the resistance specifications, replace them with new ones.
  • Reinstall the glow plugs in their original locations and tighten them securely with a wrench or socket.
  • Start your car and check to see if the glow plug light turns off. If the light stays on, follow the next step.

Step 2 Use an OBD2 scanner to identify the cause

Use an OBD2 scanner to find out the exact cause. The scanner can retrieve diagnostic trouble codes (DTCs) stored in the engine control module (ECM) related to the glow plugs.

So, how to use an OBD2 scanner:

  • Connect the scanner to the car’s OBD port.
  • Turn the ignition on.
  • Select the “Read Codes” function on the scanner to search for the codes.
  • Figure out what the code means. From the DTCs, you can also go to OBD2 Codes Lookup or the DTC Lookup Library on the scanner to know the meaning, causes, and fixes.

Step 3 Repair broken or malfunctioning parts

If you are unable to do it on your own, get your vehicle checked by a skilled technician to repair the faulty parts.

Step 4 Reset the light

Once you are done repairing, erase the faulty code from the scanner and restart your car to have a test drive. You’ll see the light is gone, and the problem has been resolved. 

What’s Next?

Now, you have a better understanding of the glow plug light and know how to respond when the light comes on.

It can be challenging to identify the core cause of the glow plug light. That’s why we need to have a scan tool.

Not sure which scanning gadget to purchase? Check out my review on the best OBD2 scanners to get a suitable product for your vehicle.

Till then, drive safely!

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Information sourced from the owner's manual.